1. Field of the Invention
The present invention relates to the use of protective agents in cancer chemotherapy in human and animal subjects. Protective agents are compounds that prevent, reduce, or otherwise ameliorate the toxic side effects of anti-cancer chemotherapeutic compounds in normal body cells while substantially preserving the anti-tumor properties of these compounds in vivo when administered prior to, concomitantly with, or subsequently to administration of such chemotherapeutic compounds. More specifically, the present invention relates to the use of D-methionine and structurally related compounds as protective agents having oto-protective, weight loss-protective, gastrointestinal-protective, neuro-protective, alopecia-protective, and survival-enhancing effects in conjunction with chemotherapy employing platinum-containing antineoplastic agents, such as cisplatin.
2. Description of Related Art
Cisplatin Chemotherapy
Cisplatin (cis-diamminedichloroplatinum(II); CDDP) is a widely used antineoplastic agent. Cisplatin administration has increased both in the variety of cancer types for which it is employed and in the amount used in a given individual to achieve maximal therapeutic effect (Blumenreich et al., 1985; Forastiere et al., 1987; Gandara et al., 1989).
The toxic side effects of cisplatin have long been recognized and are widely reported (Lippman et al., 1973; also see the review by Hacker, 1991). These toxicities include a variety of peripheral neuropathies, myelo-suppression, gastrointestinal toxicity, nephrotoxicity, and ototoxicity (Ozols and Young, 1985; Stewart et al., 1987; Stoter et al., 1989). Initially, the primary dose-limiting factor was nephrotoxicity, but now the routine administration of mannitol, hypertonic saline, and high fluid administration have ameliorated, but not eliminated, that side effect. However, ototoxicity remains uncontrolled (Bajorin et al., 1987; Fillastre and Raguenez-Viotte, 1989). Although nephrotoxicity can still be dose-limiting, currently the primary dose-limiting factor is ototoxicity (Blumenreich et al., 1985; Forastiere et al., 1987; Berry et al., 1990).
The primary ototoxic effects of cisplatin appear to occur in the cochlea. Anatomical changes occur in both the stria vascularis and the organ of Corti. The primary histologic findings include hair cell degeneration and damage to the supporting cells that are dose-related (Anniko and Sobin, 1986). At high doses, total collapse of the membranous labyrinth can occur (Anniko and Sobin, 1986). In the organ of Corti, there is loss of outer and inner hair cells, with a propensity for outer hair cell loss in the basal turn (Fleischman et al., 1975; Komune, 1981; Estrem et al., 1981; Schweitzer, 1993), and alterations in the supporting cells and Reissner's membrane (Komune, 1981; Estrem et al., 1981). Estrem et al. (1981) also reported softening of the cuticular plate and an increased number of lysosomal bodies in the apical portion of the outer hair cell. However, the mechanisms inducing these changes are largely unknown.
For equivalent inner ear concentrations, cisplatin is the most ototoxic drug known (Moroso and Blair, 1983; Koegel, 1985; Anniko and Sobin, 1986; Griffin, 1988). Generally, cisplatin ototoxicity is irreversible, its onset insidious, and the hearing loss may progress after discontinuation of the protocol (Schaefer et al., 1985; Melamed et al., 1985; Pollera et al., 1988; Aguilar-Markulis et al., 1981; see the review by Moroso and Blair, 1983). Hearing loss is usually permanent (Vermorken et al., 1983). Partial recovery may occur in some cases, but only one of 121 patients with hearing loss had complete recovery in a study by Aguilar-Markulis et al., (1981). Hearing loss typically starts at the ultra high frequencies (9000 to 20000 Hz) (Fausti et al., 1984; Kopelman et al., 1988) and then progresses into the high conventional audiometric range (Laurell and Engstrom, 1989; Kopelman et al., 1988; Meyer, 1989), reducing the patient's ability to hear consonant but not vowel sounds. An inability to understand speech and tinnitus are frequent complaints (Kopelman et al., 1988). An increasing number of patients survive chemotherapy, but frequently with hearing impairment.
Nucleophilic Sulfur Protective Agents
Many sulfur-containing compounds (including substances with thio, thiol, and thioether groups) have been reported to provide CDDP nephroprotection in animal models (Anderson et al., 1990; Jones and Basinger, 1989; Jones et al., 1986; 1991a, b, c; 1992). These compounds may act by preventing the CDDP-induced depletion of glutathione or the binding of CDDP to protein sulfhydryl groups (Hanneman and Baumann, 1988; Nakano and Gemba, 1989; Gandara et al., 1989; Ravi et al., 1991; Schweitzer, 1993).
Additionally, sodium thiosulfate (STS) and diethydithiocarbamate (DDTC) provide good CDDP otoprotection in animals (Otto et al., 1988; Church et al., 1995; Rybak et al., 1995). Unfortunately, STS may reduce CDDP tumoricidal action (Pfeifle et al., 1985; Aamdal et al., 1987) and may exacerbate CDDP-induced weight loss and mortality (Otto et al., 1988). DDTC does not interfere with antitumor action (Qazi et al., 1988; Berry et al., 1989; Dedon et al., 1984; Borch et al., 1988), but can produce severe side effects (Rothenberg et al., 1988; Qazi et al., 1988).
D-Methionine
D-methionine (D-Met) is a sulfur-containing nucleophile that provides highly effective CDDP nephroprotection in animals without decreasing anti-tumor action (Jones and Basinger, 1989). Although only tested in that single study at a single dose level, D-Met was the most effective CDDP nephroprotectant that did not interfere with CDDP tumoricidal action out of nearly 40 sulfur-containing agents tested in a series of studies by Jones and colleagues (Jones and Basinger, 1989; Jones et al., 1986; 1991a, b, c; 1992). As far as the inventor is aware, D-Met has never been previously tested as a CDDP otoprotectant, and has not yet been tested clinically (Treskes and van der Vijgh, 1993).
Sulfur-Containing Protective Agents and the Modulation of Cisplatin-Induced Toxicity
Studies indicate that individual sulfur-containing protective agents may only be effective in reducing specific types of toxicity, such as nephrotoxicity, while remaining ineffective in blocking other platinum-related complications such as peripheral neuropathy and ototoxicity (Schweitzer, 1993). In addition, an agent which is effective as a regional chemoprotector following site-specific (intraperitoneal) usage of platinum-containing compounds such as CDDP may fail to provide adequate systemic protection, or may inhibit antitumor activity (Schweitzer, 1993).
Not all sulfur-containing compounds provide protection against all of CDDP's toxicities, and it is not possible to predict which protective agents will be effective or ineffective for this purpose. For example, cefoxitin (Jones et al., 1992) does not provide nephroprotection. Ethyl-L-cysteinate and N-(2-mercapto-propionyl)glycine (Jones and Basinger, 1989) exacerbate CDDP nephrotoxicity. 2-(methylthio)nicotinic acid does not provide nephroprotection in rats (Jones et al., 1991b). The sodium salt of penicillin G does not protect against CDDP nephrotoxicity or weight loss (Jones et al., 1992). Similarly, thiamine-HCl does not protect against cisplatin nephrotoxicity or weight loss (Jones et al., 1992).
Furthermore, sulfur-containing compounds protective against one type of CDDP toxicity frequently do not protect against other CDDP toxicities, and it is not possible to predict the specific antitoxic effectiveness of such compounds. Cephalexin (Jones et al., 1992) protects against CDDP-induced kidney dysfunction and weight loss, but curiously does not prevent kidney pathology. Cefoxitin (Jones et al., 1992) provides some protection against CDDP-induced weight loss, but does not protect against CDDP nephrotoxicity. The sodium salt of penicillin G does not protect against either CDDP-induced nephrotoxicity or weight loss (Jones et al., 1992). Sulfathiazole provides protection against CDDP nephro-toxicity, but not weight loss (Jones et al., 1992).
WR2721 provides excellent CDDP nephroprotection (Mollman et al., 1988), but does not ameliorate nausea and vomiting (Glover et al., 1987). Nor does WR2721 seem to provide CDDP otoprotection: Glover et al. (1987) found mild to severe hearing loss in 20 of 36 patients receiving WR2721 prior to CDDP although nephroprotection was obtained. Rubin et al. (1995) reported a 45% incidence of significant hearing threshold shift in patients pretreated with WR2721 prior to CDDP administration. Unfortunately, neither the Glover et al. (1987) nor Rubin et al. (1995) studies employed a control group, and both reported a high incidence of ototoxicity in patients receiving WR2721. In hamsters, Church et al. (1995) reported no WR2721 protection from ototoxicity or mortality.
Even when a sulfur-containing agent is found to be protective, its side effects can be so severe that clinical applicability is precluded. In addition, even among agents that provide CDDP otoprotection, the protection may be so inconsistent and/or the side effects so great that they would not be used clinically. For example, DDTC provides protection against CDDP-induced nephrotoxicity (Qazi et al., 1988; Berry et al., 1989; Gandara et al., 1989a, 1989b, and 1991) and ototoxicity (Church et al., 1995), but the protection against ototoxicity may only be partial (Gandara et al., 1989a; Ravi et al., 1992) and its side effects are severe (Rothenberg et al., 1988; Berry et al., 1990). If DDTC dosing is reduced to ameliorate its side effects, adequate protection from CDDP side effects may not occur (Paredes et al., 1988). Similarly, disulfiram (Antabuse), which can be used as a precursor for its metabolite DDTC, can cause sensorimotor neuropathy (Argov and Mastiglia, 1979) and reversible confusion that can be dose-limiting (Stewart et al., 1987). Consequently, it is unlikely that DDTC will be widely used clinically as a CDDP chemoprotectant. In contrast, as described below, D-Met provides complete otoprotection without apparent adverse side effects.
Finally, many sulfur-containing compounds inhibit the anti-tumor action of CDDP, and it is not possible to predict which agents will or will not act in this manner. Thus, many agents that provide CDDP protection are not clinically useful. For example, Captropril (Jones et al., 1992) protects against CDDP nephrotoxicity, but reacts immediately with CDDP to form a precipitate if coadministered, thereby precluding anti-tumor efficacy. L-methioninamide (Jones et al., 1991b) provides excellent CDDP nephroprotection, but impairs CDDP anti-tumor action. Metallothionein, a sulfur-containing compound the synthesis of which is induced by administration of bismuth subnitrate, provides CDDP nephroprotection, but also inhibits CDDP anti-tumor action (Naganuma et al., 1987; Boogaard et al., 1991; Satoh et al., 1993; Imura et al., 1992; Endresen et al., 1994). STS reduces CDDP nephrotoxicity (Pfeifle et al., 1985; Howell et al. 1982) and ototoxicity (Otto et al., 1988; Church et al., 1995), although some authors report inadequate otoprotection (Markman et al., 1985). However, STS will probably not be clinically useful as coadministration with CDDP reduces the latter's tumoricidal action (Pfeile et al., 1985; Aamdal et al., 1987; Jones et al., 1991b), and two route administration does not provide nephroprotection (Jones et al., 1991b). Even in the absence of other agents, STS may also increase mortality and induce weight loss (Otto et al., 1988). Biotin, another sulfur-containing compound that provides good CDDP nephroprotection, inhibits anti-tumor activity (Jones et al., 1992).
Thus, a variety of sulfur-containing compounds can act as protective agents for particular toxicities. A comparison of C--SH-- and C--S--C-containing compounds demonstrated that the C--S--C-- group was more effective in preventing nephrotoxicity in rats (Jones et al., 1989). However, not all of the compounds possessing the C--S--C-- group were found to be effective cisplatin antagonists.
The foregoing discussion demonstrates that it is not possible to predict reliably which particular sulfur-containing nucleophile will exhibit a platinum-containing compound protective effect in any particular type of cell, tissue, or organ. Indeed, individual compounds seem to exert their protective effects only in certain tissues. Thus, the ability of any particular nucleophilic sulfur compound to act as a protective agent in any particular tissue can only be determined by direct experimentation. Of course, such compound will only be of value if it does not substantially reduce the anti-tumor efficacy of cisplatin or related anti-tumor platinum-containing compounds.
Deegan et al. (1994) demonstrated that male Wistar rats receiving a single intraperitoneal dose of cisplatin-methionine at a 1:5 ratio by weight did not exhibit cisplatin-induced nephrotoxicity. Their results indicated that cisplatin-methionine is significantly cytotoxic, yet lacks cisplatin-associated renal toxicity. These workers suggested a role for either methionine co-treatment or cisplatin-methionine compounds in the treatment of human cancers. However, they neither disclosed nor suggested the specific otoprotective, weight loss-protective, gastrointestinal-protective, neuroprotective, alopecia-protective, or survival-enhancing effects of D-methionine surprisingly discovered by the present inventor. Nor did they provide any motivation to investigate D-methionine as an otoprotectant, weight loss-protectant, survival-enhancing agent, etc., or any reasonable expectation that methionine could act in these manners during cisplatin administration. Finally, Deegan et al. provided no guidance or suggestion as to how methionine could be used as a protective agent for various toxicities in humans, as described herein. As noted by Schweitzer (1993; page 12), while various nucleophilic sulfur protective agents have been shown to be effective in blocking or reversing the renal toxicity of CDDP while retaining the chemotherapeutic activity of the drug, each agent has to be considered individually. The effects on antineoplastic activity, individual CDDP toxicities, and appropriate dosing schedules need to be determined on a per se basis for each compound.
In view of the foregoing, the utility of D-Met as a highly effective platinum-containing anti-neoplastic agent otoprotectant, weight loss protectant, gastrointestinal protectant, neuroprotectant, alopecia protectant, and survival-enhancing agent which does not interfere with anti-tumor activity, and which does not appear to cause any serious side effects, could not have been predicted. In fact, the discovery of D-Met's beneficial effects is surprising in view of the many significant problems, discussed above, encountered with previously described sulfur-containing nucleophiles that preclude their clinical use.